The synthesis, X-ray structure analysis, and photoisomerization assessment of the acyclic triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine and 1,3-Bis(biphenyl-4-yl)triazene

Article abstract of DOI:10.1007/s10870-013-0438-6

The acyclic triazenes 4-[(E)-2-(biphenyl-4-yl)diazenyl]-morpholine (1) and 1,3-bis(biphenyl-4-yl)triazene (2) were prepared and their X-ray crystal structures determined. Compound 1 crystallized in the monoclinic space group P2₁/c with a = 29.153 (5) ?, b = 6.0444 (14) ?, c = 7.5997 (15) ?, and β = 96.03 (1). Compound 2 crystallized as the 1/4 cyclohexane solvate in the triclinic space group P 1? 1 ? with a = 10.3228 (10) ?, b = 13.4541 (14) ?, c = 15.4323 (15) ?, α = 87.158 (4), β = 76.030 (4) and γ = 71.492 (5). The N-N double bond in 1 and 2 adopted an (E)-configuration and had bond angles that ranged from 112.19 (9) to 114.01 (13), which deviated from optimal trigonal planar geometry by 6 -8. A degree of π-conjugation across the acyclic triazene moieties in compounds 1 and 2 was indicated by N-N double and single bond lengths that were respectively longer and shorter than the literature values for these types of bonds. The room temperature electronic absorption spectra of 1 and 2 in various solvents were both dominated by an intense band above 300 nm that was assigned to a combination of π → π*and n → π*transitions. Compounds 1 and 2 in acetonitrile and methanol solution were found to undergo trans → cis photoisomerization. The magnitudes of trans → cis rate constants were rationalized in terms of the degree of solvent stabilization of the dipolar resonance hybrid structure of the acyclic triazene subunits in 1 and 2. Irradiation of chloroform solutions of 1 and 2 caused trans → cis photoisomerization and radical decomposition to occur. Graphical Abstract: The X-ray structures of compounds 1 and 2 were determined and their photoisomerization kinetics explored.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10870-013-0438-6

J Chem Crystallogr (2013) 43:412–420
DOI 10.1007/s10870-013-0438-6
ORIGINAL PAPER
The Synthesis, X-ray Structure Analysis, and Photoisomerization
Assessment of the Acyclic Triazenes 4-[(E)-2-(biphenyl-4-
yl)diazenyl]-morpholine and 1,3-Bis(biphenyl-4-yl)triazene
•
Carolyn Mukai Emanuel Zamora
•
•
•
Jillian L. Blatti Frank R. Fronczek
Ralph Isovitsch
Received: 27 March 2013 / Accepted: 10 July 2013 / Published online: 23 July 2013
Ó Springer Science+Business Media New York 2013
Abstract The acyclic triazenes 4-[(E)-2-(biphenyl-4-
yl)diazenyl]-morpholine (1) and 1,3-bis(biphenyl-4-yl)tria-
zene (2) were prepared and their X-ray crystal structures
determined. Compound 1 crystallized in the monoclinic
degree of solvent stabilization of the dipolar resonance
hybrid structure of the acyclic triazene subunits in 1 and 2.
Irradiation of chloroform solutions of 1 and 2 caused
trans ? cis photoisomerization and radical decomposition
to occur.
˚
space group P21/c with a = 29.153 (5) A, b = 6.0444
˚
˚
(14) A, c = 7.5997 (15) A, and b = 96.03 (1)°. Compound
2 crystallized as the 1/4 cyclohexane solvate in the triclinic
Keywords Acyclic triazenes ꢀ X-ray structures ꢀ
Electronic absorption spectra ꢀ Photoisomerization
ꢀ
˚
space group P 1 with a = 10.3228 (10) A, b = 13.4541
˚
˚
(14) A, c = 15.4323 (15) A, a = 87.158 (4)°, b = 76.030
(4)° and c = 71.492 (5)°. The N–N double bond in 1 and 2
adopted an (E)-configuration and had bond angles that ran-
ged from 112.19 (9)° to 114.01 (13)°, which deviated from
optimal trigonal planar geometry by 6°–8°. A degree of p-
conjugation across the acyclic triazene moieties in com-
pounds 1 and 2 was indicated by N–N double and single
bond lengths that were respectively longer and shorter than
the literature values for these types of bonds. The room
temperature electronic absorption spectra of 1 and 2 in
various solvents were both dominated by an intense band
above 300 nm that was assigned to a combination of
p ? p* and n ? p* transitions. Compounds 1 and 2 in
acetonitrile and methanol solution were found to undergo
trans ? cis photoisomerization. The magnitudes of
trans ? cis rate constants were rationalized in terms of the
Introduction
Acyclic triazenes are an enormous class of useful molecules
that share one common characteristic: they all contain three
successive nitrogen atoms whose bonding is similar to that
found in an allylic group, for example, RN=N–NR0R00 as
compared to H2C=CH–CR. Acylic triazenes can be prepared
using a variety of synthetic methodologies, with the usual
route being a coupling reaction between an aryl diazonium
ion and a primary or secondary amine [1, 2]. The coupling of
an aryl diazonium ion to a Grignard or alkyllithium reagent
has also been used to prepare acyclic triazenes [3]. A
recently developed synthetic methodology that gave acyclic
triazenes in high yields involved the reaction of organic
azides with neutral N-heterocyclic carbenes [4].
Research into the application of acyclic triazenes to a
wide variety of areas is ongoing [5]. The ease at which they
can be transformed into amines and their stability under a
variety of conditions makes them attractive protecting
groups for aniline derivatives [6]. Acyclic triazenes have
been used in the synthesis of novel heterocyclic com-
pounds. For example, they have served as both linker and
synthon in the solid-phase synthesis of substituted benzo-
pyridazines as well as precursors for the preparation of
aryl-1H-benzotriazoles [7, 8]. In the area of medicinal
Electronic supplementary material The online version of this
article (doi:10.1007/s10870-013-0438-6) contains supplementary
material, which is available to authorized users.
C. Mukai ꢀ E. Zamora ꢀ J. L. Blatti ꢀ R. Isovitsch (&)
Department of Chemistry, Whittier College,
13406 Philadelphia Street, Whittier, CA 90608, USA
e-mail: risovits@whittier.edu
F. R. Fronczek
Department of Chemistry, Louisiana State University,
Baton Rouge, LA 70803, USA
123

J Chem Crystallogr (2013) 43:412–420
413
chemistry the use of acyclic triazenes as cancer-fighting
drugs that function as DNA alkylating agents has been
explored [9]. Work in materials chemistry research has
utilized acyclic triazenes with unique aromatic moieties to
derivatize silicon surfaces and carbon nanotubes [10, 11].
The conjugation of acyclic triazenes and their ability to
undergo photophysical and thermal isomerization has been
exploited in the synthesis of donor–acceptor complexes
and redox-active compounds [12, 13].
Elmer Spectrum 1 FT-IR. Electronic absorption spectra
were obtained on a Hewlett Packard 8453 diode array
spectrometer. M-H-W Laboratories of Tucson, Arizona
performed the elemental analyses in this work.
Determination of Observed First Order trans ? cis
Photoisomerization Rate Constants for Compounds 1 and 2
Observed first order trans ? cis photoisomerization
rate constants (kobs) are reported as an average of three
independent trials with standard deviations. They were
obtained by monitoring the disappearance of the trans
isomer over time via the decrease in absorbance of the
longest wavelength maximum in its electronic absorption
spectrum [14]. Concentration of the trans isomer at time
t was obtained from a calibration curve of the trans isomer.
Calibration curves of the trans isomers had R2 values that
ranged from 0.9955 to 0.9973. Plots of ln [trans isomer]
versus time were linear (R2 = 0.9951–0.9997), with the
Our research seeks a better understanding of the bonding
and photophysical properties of acyclic triazenes toward
the goal of increasing their thermal stability and predict-
ably controlling their trans ? cis photoisomerization for
their eventual incorporation into photochromic devices.
Toward this end, we present the syntheses and X-ray
structures of an asymmetric and symmetric acyclic tria-
zene, both of which were prepared from 4-aminobiphenyl
(1 and 2, Scheme 1). The trans ? cis photoisomerization
behavior of 1 and 2 in various solvents is also discussed.
slopes of the lines being taken as trans ? cis kobs
.
Photoisomerization experiments were done with stirred,
room temperature, argon-bubbled solutions of 1 and 2 in a
quartz cuvette using the apparatus depicted in Fig. S1 in
the Supplementary Materials. Solutions of 1 and 2 were
approximately 2 9 10-5 M and prepared using spectropho-
tometric grade acetonitrile, chloroform and methanol. Irra-
diation was via 16 W commercially available black light
fluorescent bulbs, whose output is at wavelengths greater than
300 nm. The black light bulbs were *6 cm from the sample.
Experimental
Reagents and Techniques
Synthetic procedures were carried out using standard
techniques. Solvents and reagents were purchased from
Sigma-Aldrich or Acros Organics and used as received.
Melting points were determined in open capillaries and are
Synthesis of 4-[(E)-2-(biphenyl-4-yl)diazenyl]-
morpholine (1)
1
uncorrected. H and 13C NMR spectra were recorded on a
JEOL ECX 300 MHz spectrometer from DMSO-d6,
CDCl3, acetone-d6, and CD2Cl2 solutions using TMS as the
0.250 g (1.48 mmol) of 4-aminobiphenyl was added to 1 mL
of 6 M HCl and 2 mL of H2O, and heated for 10 min with
1
internal standard. The data from H and 13C NMR spectra
of compounds 1 and 2, respectively measured in DMSO-d6 stirring to yield an orange-pink suspension. The suspension
and CDCl3 solution, are listed below. These solvents
resulted in the best spectra in terms of larger chemical shift
difference between neighboring proton and carbon reso-
nances. Care was taken to exclude light from all NMR
samples. IR spectra were recorded as KBr disks on a Perkin
was cooled to 0 °C, diluted with 1 mL of H2O, and then an
aqueous solution of NaNO2 (0.104 g (1.55 mmol) in 1 mL of
H2O) was added dropwise over 10 min with stirring. The
reaction mixture turned a clear yellow-orange color. It was
allowed to stir for 20 min, and became slightly green. Next,
Scheme 1 The syntheses of
compounds 1 and 2
H
N
N
1. 6 M HCl, NaNO2 (aq)
N
NH2
2. 4-aminobiphenyl/CH3OH
3. sat. NaHCO3 (aq)
2
1. 6 M HCl, NaNO2 (aq)
2. morpholine
3. sat. NaHCO3 (aq)
N
N
O
N
1
123

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J Chem Crystallogr (2013) 43:412–420
0.14 mL (1.60 mmol) of morpholine was added dropwise
with stirring over 10 min. The reaction mixture formed a pale
yellow precipitate. The reaction mixture was stirred for
20 min, and 4 mL of H2O and *40 mL of 10 % aqueous
NaHCO3 were added to bring the reaction mixture to pH 8.
The crude product was collected over vacuum filtration,
washed well with H2O, and allowed to air dry. Taking care to
exclude direct light, the crude product was recrystallized
twice from a 1:3 cyclohexane-petroleum ether mixture to give
0.0798 g (31 %) of beige crystalline flakes of pure 1. MP
110–111 °C; IR (KBr pellet) 3059, 3031, 2976, 2873, 1460,
a Bruker Kappa APEX-II CCD diffractometer equipped
with an Oxford Cryosystems Cryostream chiller
and graphite-monochromated Cu Ka radiation (k =
˚
1.54178 A). For compound 1, a total of 9,226 measured
reflections with hmax = 68.5° yielded 2,392 unique data,
and for compound 2 a total of 20,672 measured reflections
with hmax = 68.8° yielded 6,852 unique data. Structures
were solved by direct methods, and structure refinements
were carried out using SHELXL-97 [15]. All H-atoms were
visible in difference maps, but were placed in idealized
positions during refinement, except for the NH hydrogen
atom in 2, for which coordinates were refined.
1
1385, 1349, 1144, 1102, 1011, 836, 761 cm-1; H NMR
(300 MHz, DMSO-d6) ppm 7.63 (d), 7.42 (d), 7.31 (m), 3.73
(m); 13C NMR (75 MHz, DMSO-d6) ppm 149.6, 140.1,
138.3, 129.5, 127.8, 126.9, 121.5, 66.1, 49.1 (br. s); EI-HR-
MS: m/z for [M?H]? = 268.1446, Calcd. m/z for
[M?H]? = 268.1450 Anal Calcd. For C16H17N3O (267.4): C
71.87, H 6.42, N 15.72. Found: C 71.99, H 6.27, N 15.83.
CCDC 891358 and 891359 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via http://www.ccdc.cam.ac.uk/
products/csd/request/.
Synthesis of 1,3-Bis(biphenyl-4-yl)triazene (2)
Results and Discussion
0.250 g (1.48 mmol) of 4-aminobiphenyl was added to 1 mL
of 6 M HCl and 2 mL of H2O and heated on a hot plate for
10 min with stirring to yield an orange-pink suspension. The
suspension was cooled to 0 °C and 1 mL of H2O added. The
suspension was maintained at 0 °C and a solution of 0.104 g
(1.55 mmol) NaNO2 in 1 mL of H2O was added drop wise
with stirring over 10 min. The reaction mixture was stirred for
20 min and turned a clear orange-yellow color. Next, a
solution of 0.275 g (1.63 mol) of 4-aminobiphenyl in 5 mL
of CH3OH was added drop wise with stirring over 10 min.
After stirring for 20 min, the reaction mixture formed an off-
white precipitate. To this, 4 mL of H2O and *40 mL of 10 %
aqueous NaHCO3 were added to bring the reaction mixture to
pH 8. Upon addition of the NaHCO3, the reaction mixture
foamed and produced a dark yellow precipitate that was
collected over vacuum filtration, washed well with H2O, and
allowed to air dry. While taking care to exclude direct light,
the crude product was recrystallized from cyclohexane.
0.153 g (59 %) of dark brown needles of pure 2 were
obtained. MP 140–141 °C; IR (KBr pellet) 3029, 2922, 2846,
Synthesis and Characterization of Compounds 1 and 2
4-Aminobiphenyl in 6 M HCl was combined with an
aqueous solution of NaNO2 to yield a diazonium ion
that underwent coupling with morpholine and with
4-aminobiphenyl to yield compounds 1 and 2, respectively
(Scheme 1) [16, 17]. Variable low to moderate yields of 1
(9–38 %) and 2 (34–59 %) were obtained, which was
explained by the diazonium ion more readily participating
in electrophilic aromatic substitution with the electron-rich
terminal ring of a neighboring biphenyl moiety rather than
undergoing coupling [18].
Different analytical techniques were used to confirm the
identity of 1 and 2. While the NMR spectra of both com-
pounds offered valuable insights into their structures, such
as the resonances at 66.1 and 49.1 ppm in the 13C NMR
spectrum of 1 that were assigned to the morpholine car-
bons, and the singlet at 9.78 ppm in the 1H NMR spectrum
of 2 that was assigned to the proton of the acyclic triazene
subunit, they did exhibit anomalous behavior [19, 20].
1
1608, 1532, 1484, 1430, 1384, 1252, 1205, 765, 698 cm-1
;
The H NMR spectra of both compounds had integrals
1H NMR (300 MHz, CDCl3) ppm 9.78 (s), 7.63(m), 7.49(m),
7.36 (m); 13C NMR (75 MHz, CDCl3) ppm 144.4 (br. s)
140.6, 139.8, 129.0, 128.1, 127.3, 127.0, 118.0 (br. s); EI-HR-
MS: m/z for [M?H]? = 350.1647, Calcd. m/z for
[M?H]? = 350.1657 Anal Calcd. For C24H19N3 (349.5): C
82.48, H 5.49, N 12.03. Found: C 82.37, H 5.60, N 11.88.
that were different from the predicted values by
1
hydrogen atom. This observed discrepancy may be caused
by different rotamers and conformers of 1 and 2 that exist
in solution. Both of these phenomena would cause the
hydrogen atoms of compounds 1 and 2 to be in slightly
different environments, resulting in integrals that were
slightly off from the expected values [17, 18, 21]. The
1
X-ray Structure Determination
integrals in the H NMR spectra of both compounds were
observed to be lower or higher than expected when
obtained from DMSO-d6, CDCl3, acetone-d6, and CD2Cl2
solution.
X-ray quality crystals of 1 and 2 were used for data col-
lection at T = 90 K. The data for 1 and 2 were collected on
123

J Chem Crystallogr (2013) 43:412–420
415
The existence of rotamers and conformers of 1 and 2 in
solution was supported by the presence of broad singlets in
the 13C NMR spectra of both [17, 21]. The missing reso-
nance in the 13C NMR spectrum of 1 was rationalized in
terms of the signals from two carbon atoms with small
chemical shift differences being obscured by a broadened
peak.
ones for each compound. Compounds 1 and 2 both yielded
acceptable elemental analyses.
Crystallographic Study
Crystal and structure refinement data for compounds 1 and
2 are presented in Table 1, while selected bond lengths and
angles for both compounds are in Table 2. The ORTEP
representation of 1 is shown in Fig. 1, and that of 2 is
shown in Fig. 2.
High-resolution mass spectroscopy and elemental anal-
ysis of 1 and 2 supported their identity and purity. Both
compounds had HR–ESI–MS that exhibited [M?H]?
peaks at m/z = 268.1446 and 350.1647, respectively.
These values were both within 3 ppm of the calculated
The asymmetric unit of compound 2 consisted of two
independent molecules of 2 that had essentially equal bond
Table 1 Crystal and
experimental data for
compounds 1 and 2
Compound
1
2
Empirical formula
Formula weight
CCDC deposition no.
Space group
C16H17N3O
267.33
C24H19N3ꢀ0.25(C6H12
370.46
)
891358
891359
ꢀ
Triclinic, P 1
Monoclinic, P21/c
Crystal dimension (mm3) 0.34 9 0.33 9 0.04
0.30 9 0.20 9 0.15
Unit cell parameters
˚
˚
10.3228 (10) A
a
b
c
a
b
c
29.153 (5) A
˚
6.0444 (14) A
˚
13.4541 (14) A
˚
7.5997 (15) A
˚
15.4323 (15) A
87.158 (4)°
76.030 (4)°
71.492 (5)°
96.03 (1)°
3
˚
3
˚
V
1331.8 (5) A
1971.5 (3) A
Temperature (K)
90
90
Z
4
4
Dc (Mg m-3
)
1.333
1.248
Radiation
l (CuKa) (mm-1
CuKa
0.68
CuKa
)
0.57
F(000)
568
784
hmax
68.5°
68.8°
Index ranges
-31 B h B 34
-7 B k B 7
-8 B l B 9
0.033
-10 B h B 11
-16 B k B 15
-18 B l B 18
0.027
Rint
Goodness of fit on F2
Reflections collected
Independent reflections
1.06
1.02
9,226
20,672
6,852
2,392
Reflections with
I [ 2r(I)
2,262
5,763
Refined parameters
R1/wR2
182
530
0.034/0.088
0.041/0.109
Measurement
Refinement
Bruker APEX-II CCD diffractometer Bruker APEX-II CCD diffractometer
Full-matrix least squares method on
F2
Full-matrix least squares method on
F2
Extinction coefficient
0.0014 (2)
–
-3
-3
˚
0.23, -0.19 e A
˚
0.24, -0.17 e A
(Dq)max, (Dq)min
123

416
J Chem Crystallogr (2013) 43:412–420
˚
with a N–N double bond (N1–N2) length of 1.2891 (18) A
˚
and a N–N single bond (N1–N2) length of 1.3109 (19) A.
lengths and angles, as well as one-half of a molecule of
cyclohexane.
The acyclic triazene subunits of compounds 1 and 2
were found to have similar structural characteristics. The
N–N double bonds in 1 and 2 adopted an (E)-configuration.
The bond angles of the N–N double bond in both com-
pounds deviated from optimal trigonal planar geometry by
6°–8°. For example, compound 1 had a N3–N2–N1 bond
angle of 113.78 (9)° and a N2–N3–C5 bond angle of
112.19 (9)°, and compound 2 had a N1–N2–N3 bond angle
of 114.01 (13)° and a N2–N1–C1 bond angle of 113.32
(13)°. The geometry and bond angles of the N–N double
bond in 1 and 2 are similar to those of related acyclic
triazenes [22, 23].
Compound 2 had a greater degree of delocalization than
compound 1 in its acyclic triazene moiety, as indicated by
its more similar N–N bond lengths.
Additional evidence for p-conjugation in the triazene
subunits of 1 and 2 was observed. The bond angles of the
morpholine nitrogen atom (N1) in 1 were observed to be
111.84 (8), 114.64 (8) and 119.39 (9)°. These bond angles
indicate the partial sp2 character of N1 and its participation
in p-delocalization with N2–N3. Conjugation of the tria-
zene moiety (N1–N2–N3) with a phenyl ring (C1–C6) in 2
˚
was indicated by the 1.415 (2) A length of N1–C1, which
˚
was somewhat shorter than the literature value of 1.431 A
Deviations from the literature bond length values for a
for a Caryl–N bond [24]. p-Delocalization of the type
observed in compounds 1 and 2 has been observed in
similar acyclic triazenes [25–27].
2
N–N double bond (1.222 A) and a N(sp )–N(sp ) single
3
˚
˚
bond (1.420 A) indicated a degree of p-conjugation across
the acyclic triazene moieties in compounds 1 and 2 [24].
For example, the N–N double bond in 1 (N2–N3) was
˚
longer than expected with a length of 1.2660 (14) A.
In general, the bond lengths and angles of compounds 1
and 2 corresponded well to related compounds in the lit-
erature [19, 22–28].
Conversely, the N–N single bond (N1–N2) was shorter
˚
than expected with a length of 1.3620 (13) A. The same
The packing interactions of 1 are show in Fig. 3.
Crystals of 1 were composed of stacks of individual mol-
ecules of 1 engaged in short-contact NꢀꢀꢀH and pꢀꢀꢀp van
der Waal’s interactions. Neighboring stacks of 1 partici-
pated in weak intermolecular C–HꢀꢀꢀO hydrogen bonding
between a morpholine hydrogen atom (C1–H1A) and a
morpholine oxygen atom (O1) with a HꢀꢀꢀO distance of
deviations from the ideal were observed in compound 2
˚
Table 2 Selected bond lengths (A) and angles (°) for compounds 1
and 2
1
˚
˚
*2.57 A, a C1–O1 distance of 3.507 (2) A, and a C1–
H1AꢀꢀꢀO1 bond angle of 158° [29].
C12–C13
C8–C11
C6–C7
1.3880 (16)
1.4886 (15)
1.3831 (16)
1.2660 (14)
1.4686 (14)
1.4341 (14)
112.19 (9)
111.84 (8)
109.21 (9)
114.64 (8)
C11–C12
C7–C8
1.3999 (16)
1.3965 (17)
1.4306 (14)
1.3620 (13)
1.5164 (15)
120.60 (11)
113.78 (9)
119.39 (9)
111.04 (9)
The main packing interactions of compound 2 involved
two molecules of 2 participating in intermolecular N–HꢀꢀꢀN
hydrogen bonding via their triazene subunits. The first
hydrogen bond occurred between N1 and the hydrogen
N3–C5
N2–N3
N1–N2
N1–C3
C3–C4
˚
bonded to N4, and had a HꢀꢀꢀN distance of 2.14(2) A, a
O1–C4
C13–C12–C11
N3–N2–N1
N2–N1–C2
O1–C4–C3
˚
N1ꢀꢀꢀN4 distance of 3.036 (2) A, and a N1ꢀꢀꢀH4N–N4 bond
N2–N3–C5
N2–N1–C3
N1–C3–C4
C2–N1–C3
2
angle of 175.7 (16)°. The second occurred between N6 and
the hydrogen bonded to N3, and had a HꢀꢀꢀN distance of
˚
˚
2.24(2) A, a N6ꢀꢀꢀN3 distance of 3.153 (2) A, and a
N1ꢀꢀꢀH4N–N4 bond angle of 171.9 (16)° [30]. The two
molecules thus formed a hydrogen-bonded dimer of graph
set R22(8) [31].
C8–C9
1.377 (2)
C7–C8
1.402 (2)
C4–C7
1.473 (2)
C2–C3
1.377 (2)
C1–C2
1.386 (2)
N1–C1
1.415 (2)
Electronic Absorption Spectroscopy
N1–N2
1.2891 (18)
1.416 (2)
N2–N3
1.3109 (19)
1.387 (2)
N3–C13
C31–C36
C29–C30
N4–C25
N5–N6
C35–C36
C28–C31
C25–C30
N4–N5
NMR samples of compounds 1 and 2 in DMSO-d6, CDCl3,
acetone-d6, and CD2Cl2 solution were observed to undergo
color changes when left out on the bench top and exposed to
ambient light in the laboratory over a period of 24 h. This
observation prompted an investigation of the electronic
absorption spectra of both compounds in a polar aprotic, a
polar protic, and moderately polar solvent (acetonitrile,
methanol and chloroform, respectively). Electronic absorp-
tion data for compounds 1 and 2 are summarized in Table 3.
1.396 (2)
1.491 (2)
1.383 (2)
1.392 (2)
1.4155 (19)
1.2898 (18)
113.32 (13)
117.28 (14)
120.93 (16)
114.13 (13)
1.3070 (19)
1.422 (2)
N6–C37
N2–N1–C1
N2–N3–C13
C9–C8–C7
N6–N5–N4
N1–N2–N3
C3–C2–C1
N5–N4–C25
N5–N6–C37
114.01 (13)
121.18 (15)
116.93 (13)
113.35 (13)
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J Chem Crystallogr (2013) 43:412–420
417
Fig. 1 ORTEP view of
compound 1. Ellipsoids are
represented at the 50 %
probability level
Fig. 2 ORTEP view of compound 2 with intermolecular N–HꢀꢀꢀN hydrogen bonding emphasized. Ellipsoids are represented at the 50 %
probability level
Compounds 1 and 2 have electronic absorption spectra that
were insensitive to solvent polarity and hydrogen-bonding
ability. A single, intense band dominated the spectra of both
compounds. In compound 1, this band was at *330 nm
(e = 18,600–20,000 L mol-1 cm-1) and in compound 2 at
*375 nm (e = 25,400–28,300 L mol-1 cm-1). The inten-
sities of these bands, as indicated by their molar extinction
coefficients, suggested that they arise from a p ? p* transi-
tions [32, 33]. A recent computational investigation by Jac-
quemin and coworkers on a series of acyclic triazenes
revealed that the origin of this band is more likely a
combination of p ? p* and n ? p* transitions [34]. They
indicated that these two transitions would be observable in the
spectra of acyclic triazenes that were unsymmetrically
substituted with groups of differing electron-donating or
accepting character as a broadened asymmetric band at
wavelengths greater than 300 nm. This is the case with
compound 1, with its maximum at *330 nm and a shoulder
at 305 nm (e = 17,600–19,100 L mol-1 cm-1). The spectra
of compound 2 had less intense absorption bands below
310 nm, which were assigned to p ? p* transitions based on
the biphenyl unit [35].
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J Chem Crystallogr (2013) 43:412–420
(k [ 300 nm) of solutions of 1 and 2 resulted in the gen-
eration the cis isomer [1]. The clear isosbestic points that
manifested in the electronic absorption spectra taken dur-
ing the irradiation of 1 and 2 in acetonitrile and methanol
support trans ? cis photoisomerization as the major pro-
cess. Based on their electronic absorption spectra, thermal
isomerization of the cis isomer of compounds 1 and 2 back
to the trans isomer was not observed at ambient conditions.
Irradiation of solutions of 1 and 2 in acetonitrile and
methanol led to a decrease in absorbance above 300 nm,
and an increase at *260 nm. The clean separation of these
two areas permitted the quantification of the concentration
of the trans isomer at various times t during irradiation.
From these data, the trans ? cis kobs could be determined
[14]. The processes occurring in irradiated chloroform
solutions of 1 and 2 were observed to be more complex.
Table 4 summarizes the first order, observed trans ?
cis kobs for 1 and 2 in the aforementioned solvents. The
changes that occur in methanol solutions of compounds 1
and 2 during irradiation are illustrated in Figs. 4 and 5.
These plots respectively depict the electronic absorption
spectra of 1 and 2 at various times t during the course of a
photoisomerization experiment. Similar plots of acetoni-
trile and chloroform solutions of 1 and 2 are shown in
Figs. S2–S5 in the Supplementary Materials.
Fig. 3 Crystal packing in compound 1 as viewed along the crystal-
lographic b-axis, with intermolecular C–HꢀꢀꢀO hydrogen bonding
emphasized
Table 4 Observed first order photoisomerization (trans ? cis) rate
constants for compounds 1 and 2 in various solvents
Table 3 Electronic absorption spectral data for compounds 1 and 2
in various solvents
1
2
kmax,abs (nm) (e)a
CH3CN
CHCl3
0.013 0.001
0.034 0.004
Photolytic decomposition
0.045 0.004
Photolytic decomposition
0.0026 0.0003
1
2
CH3OH
CH3CN
CHCl3
328 (19,400)
305 (18,400)b
375 (28,200)
304 (11,600)
267 (15,200)
375 (25,400)
304 (10,400)
266 (17,900)
377 (28,300)
304 (11,300)
267 (17,300)
Rate constant unit s-1
333 (18,600)
305 (17,600)b
CH3OH
325 (20,000)
305 (19,100)b
Spectra were acquired at room temperature unless otherwise noted
a
Molar extinction coefficients L mol-1 cm-1
b
Shoulder
trans ? cis Photoisomerization Studies
While the trans ? cis photoisomerization kinetics of
compounds similar to 2 have been investigated, those of
compounds related to 1 have not [36, 37]. Irradiation
Fig. 4 Electronic absorption spectra of the irradiation of a dilute
CH3OH solution of compound 1 taken at 1-min intervals from 0 to
20 min. Isosbestic points were observed at 235 and 271 nm
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J Chem Crystallogr (2013) 43:412–420
419
acetonitrile can form dipole–dipole interactions with the
sterically crowded and partially positive tertiary nitrogen.
The order-of-magnitude difference between the
trans ? cis kobs for compound 2 in acetonitrile and
methanol solutions can be understood again in terms of
solvent stabilization of the dipolar resonance hybrid.
Methanol provided the most stabilization as it is able to
hydrogen bond with both ends of the hybrid, while aceto-
nitrile can only form dipole–dipole interactions with the
positive end.
The irradiation of a chloroform solution of 1 for 1.5 min
resulted in the photobleaching of its absorbance at 333 nm.
The irradiation of a chloroform solution of 2 behaved
similarly to the irradiations of 2 in acetonitrile and meth-
anol, with its electronic absorption spectra manifesting an
isosbestic point at 296 nm. The data from both of these
photoisomerization experiments did not behave in a linear
fashion, which was attributed to minimal, if any, solvent
stabilization of the acyclic triazene linkage. This lack of
stabilization is theorized to have resulted in a combination
of trans ? cis photoisomerization and radical decompo-
sition processes occurring during irradiation [1, 33].
Fig. 5 Electronic absorption spectra of the irradiation of a dilute
CH3OH solution of compound 2 take at 4-min intervals from 0 to
40 min. Isosbestic points were observed at 237 and 301 nm
N
B
N
N
N
N
N
A
δ+
N
Conclusions
N
N
δ−
Two acyclic triazenes, compounds 1 and 2, were synthe-
sized and their X-ray structures determined. Similarities in
the acyclic triazene subunits of compounds 1 and 2 were
revealed by their X-ray structures. The N–N double bond in
both compounds adopted an (E)-configuration and had
similar bond angles around the atoms in their acyclic
triazene moieties. Both compounds had a degree of delo-
calization across their acyclic triazene linkages based on
N–N double and single bond lengths that were respectively
longer and shorter than those values found in the literature.
The more similar N–N bond lengths of compound 2 indi-
cated that it had a greater degree of delocalization than
compound 1 in its acyclic triazene unit. The packing
interactions of 1 consisted of short-contact van der Waal’s
interactions and intermolecular C–HꢀꢀꢀO hydrogen bond-
ing. The predominant packing interactions of 2 involved
two molecules of 2 participating in intermolecular N–HꢀꢀꢀN
hydrogen bonding. The electronic absorption spectra of
both compounds 1 and 2 were insensitive to solvent
polarity and hydrogen bonding ability, and were dominated
by an intense band above 300 nm that arose from a com-
bination of p ? p* and n ? p* transitions. Both com-
pounds 1 and 2 in methanol and acetonitrile solution
underwent trans ? cis photoisomerization, with the rate of
this process dependent on the extent of solvent stabilization
of the dipolar resonance structure of the acyclic triazene.
Irradiation of compounds 1 and 2 in chloroform solution
resonance hybrid
Scheme 2 Resonance representation of the delocalization in the
acyclic triazene subunits of compounds 1 and 2
Resonance structures A and B and their hybrid are
shown in Scheme 2 and illustrate the delocalization of the
acyclic triazene subunit of compounds 1 and 2. The trends
in trans ? cis kobs can be rationalized in terms of the
stabilization of the dipolar hybrid through intermolecular
interactions with the solvents used in the irradiation
experiments. More effective solvent stabilization of the
hybrid results in a greater degree of delocalization and a
decreased probability that the triazene will undergo
trans ? cis photoisomerization [1, 38, 39]. Solvent effects
were seen to be more important than substituent effects as
both ends of the acyclic triazene subunits in 1 and 2 were
substituted with similar electron-donating groups.
The difference between the trans ? cis kobs for com-
pound 1 in acetonitrile and methanol solutions was slight,
with the former approximately three times smaller than the
latter. This was rationalized in terms of both solvents only
being able to effectively engage in intermolecular inter-
actions with one end of the dipolar acyclic triazene reso-
nance hybrid. Methanol can form hydrogen bonds with the
partially negative end, while the linear cyano group of
123

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J Chem Crystallogr (2013) 43:412–420
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kinetics.
Acknowledgments Whittier College is acknowledged for support
of this research. The upgrade of the diffractometer was made possible
by Grant No. LEQSF (2011–2012)-ENH-TR-01, which was admin-
istered by the Louisiana Board of Regents. Dr. Simon Jones is
thanked for providing HR–ESI–MS of compounds 1 and 2.
¨
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